1. Field of the Invention
The present invention relates generally to an electrophotographic photoreceptor and its manufacturing method utilized particularly in a copying machine, a printer, or a facsimile machine and used when an imaging operation is performed with an electrophotographic process.
2. Description of the Related Arts
As is conventional, an imaging system using an electrophotographic photoreceptor forms a toner image on a surface of the photoreceptor by forming an electrostatic latent image with a laser exposure and by developing the image with toner particles to render the image visible after the surface of the photoreceptor with photoconductive property is charged by the corona discharge. The photoreceptor is composed of a conductive support and a photoconductive layer formed on the conductive support and consists of a charge generating layer and a charge transporting layer. The toner image on the photoreceptor is transferred on a recording medium by a transferring device.
In an electrophotographic process described above, a semiconductor laser (with a wavelength of 650 to 820 nm) generally used as a light source during laser exposure has a monochromatic light with a coherent property. In this case, an interference may be occurred between incident lights being incident on the surface of the photoreceptor from the semiconductor laser and reflected lights produced by reflection of lights on the surface of the conductive support which lights have transmitted through the photoconductive layer on the surface of the photoreceptor. As a result of its interference, an image defect referred to as interference fringe (moire fringe) pattern may be occurred. In particular, when a high gradient is required in a halftone and solid image and an image with horizontal ruled line patterns, there was a problem in which a defect in the electrostatic latent image was apt to occur which might become a cause of the moire fringes when a latent image was formed with a laser.
The cause of this interference will be explained hereinafter. That is, the charge generating layer in the photoconductive layer generates carriers according to the absorption of light, however, the charge generating layer tends to be made thin in order to shift the carriers generated to the charge transporting layer smoothly. Therefore, a quantity of light which is not absorbed in this charge generating layer transmits through the charge generating layer and is reflected on the surface of the conductive support. Its reflected light is believed to make interference with the reflected light from and the incident light onto the surface of the photoconductive layer, resulting in nonuniformity of contrast with the interference fringe pattern.
As prevention measures of this interference fringes, it is known that methods of expecting the scattering effect such as a method of including a light scattering substance in a underlying layer (Japanese Patent Laid-Open No. Sho 57-165845) and subjecting the surface of the conductive support to the burnishing process (Japanese Patent Laid-Open No. Hei 3-149180) or to the sand-blasting process (Japanese Patent Laid-Open No. Sho 57-16554), a method of increasing the absorbance in the charge generating layer to make the reflected light faint, and a method of making the surface of the conductive support rough in moderation (Japanese Patent Laid-Open Nos. Sho 60-186850, Sho 60-225854, Sho 60-252359, and Sho 60-256153).
Further, what causes problems in an electrophotographic photoreceptor is that a local electrification defect based on an imperfection of the photoreceptor, and the defect often causes a conspicuous image failure such as black spots and fogs. Various reasons can be considered for causing the local electrification defect, and many of the reasons are considered such that the defect is based on the local charge injection between the conductive support and the photoconductive layer.
Many of the conductive support use aluminum or aluminum based alloy. On the other hand, it can be considered to provide a blocking layer between the conductive support and the photoconductive layer in order to improve the problem of the interference. As the blocking layer, as is conventional, there is a method of providing a resin layer, such as polyamide, polyimide, polyvinyl alcohol, polyurethane, casein, or cellulose, or an inorganic layer, such as aluminum oxide, aluminum hydroxide, or the like. An inorganic layer, that is, an anodic oxidation film, is itself a homogeneous film without a pin-hole, however, the homogeneity of the film is dependent on compositions of the conductive support, because aluminum ions of the conductive support are consumed during anode oxidation treatment. If crystallized particles exist on the conductive support, recesses referred to as pits cause the surface to be uneven, thereby not only affecting manufacture of the photoconductive layer, but also causing an image defective. Therefore, in view of preventing the interference fringes described above, it was essential to control the surface figuration of the finish condition of the photoreceptor.
Some amount of Mg, Si, Cu, Ti, or the like is added to an aluminum alloy used in the conductive support in order to keep a constant strength, however, impurities such as Fe and Mn derived from an aluminum base metal are also included in the aluminum alloy. These elements form crystallized particles in the course of making an aluminum alloy to be ingot and shaping it to a tubular conductive support. These crystallized particles have chemical properties different from that of aluminum, so that they were dissolved antecedently in the anode oxidation treatment to cause the crystallized particles in the neighborhood of the surface to be left out, resulting in generation of pits.
As mentioned above, there were following problems in the prior art.
(1) A method of providing an asperity process (rough surface process) on a surface of the conductive support is typically used as it can obtain the prevention effect for interference fringes independent of the configuration of the photoconductive layer. To provide specific asperities on the surface of the conductive support can expect some amount of the prevention effect due to the light scattering effect as such. However, the complete cancellation of the interference fringes can not be achieved because factors of reflected lights still exist. Further, there were problems such that risk of increasing charge injection from heights made too rough, and also ground fog tends to occur, particularly in white solid printing.
(2) Many rough surface processes conduct the burnishing process or the sand-blasting process as the second processing, after a surface of the conductive support is once subject to the cutting process, so that the productivity was very wrong, therefore, these processes were not suitable for mass production.
In addition, qualitatively in processing, a periodical processing pattern tends to be formed on the surface of the conductive support. In particular, when surface roughness Ry (when a reference length of 0.8 mm is measured in JIS Standard) becomes larger than 2.0 .mu.m, swell and flocculation state on a coating film of the photoconductive layer would be generated, so that not only coating irregularity is apt to be produced, but also stripe like noise becomes a large problem. On the contrary, when Ry (when a reference length of 0.25 mm is measured in JIS Standard) is less than 0.8 .mu.m, in a case of a photoreceptor, problems such as a light interference and an excessive exposure phenomenon due to a laser would tend to be occurred.
Incidentally, in JIS Standard, the reference length during the measurement of the roughness is a reference value for measuring the roughness of a measurement object, and it shows a length of an interval including an arbitrary number of top and bottom portions in a roughness waveform in which tops and bottoms of the roughness periodically come out. When the roughness is large, the reference length of 0.8 mm measurement is conducted, and when it is small, the reference length of 0.25 mm measurement is performed. A result measured (value of Ry) corresponds to a peak value of a waveform in which high frequency portions in the roughness waveform are eliminated, that is, the maximum height in projection portions on a surface of the conductive support.
(3) Defectives such as pits or the like can not be prevented even if a high purity aluminum alloy is used for the conductive support. In addition, a method for decreasing the defectives in the process of the anode oxidation treatment can also not prevent the change of crystallized particles already formed in the course of making an aluminum alloy to be ingot and shaping it to be tubular. This method uses high purity aluminum alloy and requires a high precision rectifier operation for current, forcing a product to be expensive.
Further, the blocking layer produced using an anodic oxidation film and a macromolecular resin obtained by these methods would be difficult to eliminate an image defective such as black spots and fogs, and in particular, multiple fogs would be generated under the high temperature and humidity environment.
(4) On the other hand, the conventional method using an anodic oxidation film has disadvantages such that variability tends to be generated in the blocking effect and heat-resistant characteristic is poor. Accordingly, problems would be encountered concerning generation of cracks on the surface during drying process, or coating irregularity, deterioration of electric insulation breakdown strength, or growth of cracks into the photoconductive layer during formation of the photoconductive layer. Therefore, it is necessary to form an anodic oxidation film without such drawbacks.